Conventionally cases for electronic computers and communication equipments are made of iron materials and the surfaces of the cases finished by a surface treatment, such as galvanizing, nickel plating or coating with a conductive paint, for electromagnetic shielding and electrostatic shielding.
On the other hand, a light material produced by coating the surface of an aluminum material with a highly corrosion-resistant aluminum oxide film has become known by the trade name "ALUMITE". A technique of nickel plating an aluminum oxide film formed over the surface of an aluminum material for making the aluminum oxide film conductive has been published ("Electrodeposition of Nickel and Zinc in Microscopic Pores in Anode-oxidized Aluminum Films", Fukuda and Fukushima, Kinzoku Zairyo Gijutsu Kenkyu-sho, Kinzoku Hyomen Gijutsu, 33, 5 (1982)). According to this published paper, ten to twenty minutes after forming a carbon electrode and a galvanic cell by applying a voltage of 20 V for thirty minutes to an aluminum material dipped in a 98 g/l sulfuric acid solution of 30.degree. C, decreasing the voltage from 20 V to 0.08 V in four minutes, and maintaining the voltage at 0.08 V for thirteen minutes, the aluminum material is electroplated with nickel at a current density of 0.5 A/dm.sup.2.
In plating cases for electronic computers or the like by a conventional plating technique, faulty plating is liable to occur in the inner corners of square structures, such as square pipes. Galvanized cases have problems in that whiskers, namely, hairly crystals, grow with time and the whiskers short-circuit the electronic parts contained in the cases. Coatings of conductive paint are incapable of high corrosion resistance, and allow rusting and the adhesion of waste fibers and dust in the environment onto the surface of the coated cases, and entail troubles attributable to conductive waste fibers falling on the electronic parts contained in the coated cases.
The above-mentioned known method of nickel-electro-plating an aluminum oxide film requires a long plating time, and is incapable of forming a practically satisfactory corrosion-resistant and conductive film due to sporing, namely, a phenomenon in which the explosion of hydrogen occurs in minute pores in the aluminum oxide film during the plating process.
It is the principal object of the present invention to solve the above-mentioned problems, to provide a method of forming a composite film over the surface of aluminum materials by forming an aluminum oxide film over the surface of aluminum materials and plating the aluminum oxide film with nickel in a short plating time without entailing sporing, to produce a practically applicable, corrosion-resistant, conductive light member, and to enable the application of this member for constructing cases for electronic computers.
The contacts and terminals of electronic parts are formed of metals, such as aluminum, and are plated with gold to reduce the resistance to the least possible extent. In the conventional gold-plating process, the surface of an aluminum material is plated with nickel by an ordinary process, and then the nickel-plated surface is plated with gold. In the gold-plating process, the aluminum material as a cathode and soluble gold as an anode are immersed in a gold cyanide bath, and the aluminum material and the soluble gold are connected to a DC power supply for gold-plating.
In the conventional method of gold-plating the surface of an aluminum material, defects in the plated film, such as blisters, are liable to be caused by pin holes and other defects in the surface of the aluminum material, and a large amount of gold must be deposited over the surface of the aluminum material to provide the surface with a satisfactory conductivity, which increases the cost of plating the aluminum material.
Furthermore, the above-mentioned known method of electroplating an aluminum oxide film with nickel requires a long plating time, and has difficulty in practical application due to its tendency to cause sporing, namely, the explosion of hydrogen gas in the minute pores in the aluminum oxide film.
It is another object of the present invention to solve the above-mentioned problems and to provide a method of gold-plating aluminum materials using a lesser amount of gold and capable of forming an nondefective plated gold film, in which a corrosion-resistant, conductive composite film of oxide aluminum and nickel is formed over the surface of an aluminum material in a short plating time without causing sporing, the composite film is gold-plated, and then pores in the aluminum oxide film are sealed.
In order to construct cases for electronic equipment in a light-weight construction and to harden the surface of such cases, an aluminum material coated with a hard anodic oxidation coating of chromium or a hard anodic oxidation coating of chromium is used for constructing the cases.
The conventional aluminum member coated with a hard anodic oxidation coating cannot be coated with a hard paint coating. Accordingly, the plated surface appears only in the intrinsic color of the plated chromium or rhodium, namely, chrome black or the color of chromium, or the color of rhodium, and hence it is impossible to finish the surface of the hard member in a desired color.
Furthermore, the above-mentioned known method of electroplating an aluminum oxide film with nickel requires quite a long plating time and is subject to sporing, namely, the explosion of hydrogen gas in minute pores in the aluminum oxide film, during the plating process, and hence the practical application of this known method has been difficult.
It is a further object of the present invention to solve the above-mentioned problems and to provide a method of dyeing a hard anodic oxidation coating, capable of dyeing the plated surface of aluminum materials in a desired color, in which a corrosion-resistant, conductive, composite film of aluminum oxide and nickel is formed in a short plating time without causing sporing, a hard anodic oxidation coating is formed over the aluminum material coated with the composite film, and then the aluminum material coated with the composite film and the hard anodic oxidation coating is immersed in a dye solution.